ARM: keystone2: Only link cmd_ddr3.o on non-SPL builds

When we switch to including all linker lists in SPL it is important
to not include commands as that may lead to link errors due to other
things we have already discarded.  In this case simply move cmd_ddr3.o
over to the list with the rest.

Cc: Vitaly Andrianov <vitalya@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>